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Quantum breakthrough at DTU Physics

Squeezed light improves feedback cooling considerably

Researchers from DTU Physics have for the first time ever succeeded to conduct an experiment that demonstrates that squeezed states of light can enhance the feedback cooling more efficient than is possible with conventional laser light. It is an important step toward being able to manipulate mechanical objects in quantum experiments at room temperature.

Squeezed light: Heisenberg's uncertainty principal sets a limit to how to accurately know a laser beam amplitude and phase simultaneously. The uncertainties in the two parameters might be very different, but the product of them will always be greater than a certain limit.

By squeezing the light, a quantum correlation is introduced between the photons making it possible to channel the vast majority of the uncertainty of either amplitude or phase, depending on which parameter it is relevant to the measurement in the experiment. By squeeze light phase, researchers can carry out interferometric measurements with a sensitivity that is much better than with the conventional laser light.

It is well known that it is possible to cool the vibrations out of a mechanical oscillator - an everyday example would be a sling - by means of a damping feedback. However, the conducted experiments demonstrating the effectiveness of this technique is increased considerably when modern quantum technology is applied in the form of squeezed light.

"Feedback cooling is comparable to the mechanism we know from e.g. a tightrope walker that adjusts and finds the balance based on knowledge of the body's center of gravity and the continuous input from the rope swings," explains Ulrik Busk Hoff. "When we use quantum technology we so to speak sharpen the tightrope and the senses of the tightrope walker and optimize the reactions so that the walker can effectively minimize the linens fluctuations".

"The successful experiment has given us the courage to examine whether we can apply feedback technology on other mechanical objects and in more complicated experiments. The initial success is only the first step on our journey towards being able to carry out experiments that make it possible to bring a mechanical object in a simultaneous state of two very different variations. We will be testing the Austrian scientist Erwin Schrödinger´s thought experiment from 1935 that cat might be both alive and dead at the same time - using a tiny trampoline, which we will try to get to bow both upwards and downwards at the same time. If we succeed, we will have made a significant contribution to basic research in quantum technology, which is a huge focus area in Denmark these years, "explains Ulrich Busk Hoff.

The experiments specifically worked with a small circular resonator, mikrotoroid, just 60 microns high. The vibrations of the mikrotoroid were measured both by ordinary laser light and with the involvement of quantum technology. The measurements gave the real-time signal used for applying a feedback and thus cooling the object. It showed that the temperature of the mechanical object reduces by 140 degrees with respect to the environment at room temperature, when using squeezed light. It was an improvement of 12% compared to what is achievable with conventional laser light.

Quantum technology is one of tomorrow's major strategic research initiatives not only in Denmark but also in rest of the EU. Denmark hold a leading position in several areas that could ultimately affect, for example, secure encryption of data, contribution to the development of highly sensitive sensors and more accurate medical diagnostics.

"DTU Physics are among the leaders when it comes to squeeze light and has among other things the former "world record holder" employee. It is also nice that we are the first in the world to succeed in the experiment, "adds Ulrich Busk Hoff.